AV nodal stimulation during atrial tachyarrhythmia to prevent inappropriate therapy delivery

ABSTRACT

The disclosure describes techniques for delivering electrical stimulation to decrease the ventricular rate response during an atrial tachyarrhythmia, such as atrial fibrillation. AV nodal stimulation is employed during an atrial tachyarrhythmia episode with rapid ventricular conduction to distinguish ventricular tachyarrhythmia from supraventricular tachycardia and thereby prevent delivering inappropriate therapy to a patient.

This application is a continuation of U.S. patent application Ser. No.15/256,091 filed Sep. 2, 2016, now U.S. Pat. No. 10,786,678, which is acontinuation of U.S. patent application Ser. No. 13/105,689 filed May11, 2011, now U.S. Pat. No. 9,433,791, the entire content of eachapplication is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to medical devices and, more particularly, tomedical devices that deliver electrical stimulation therapy.

BACKGROUND

When functioning properly, a heart maintains its own intrinsic rhythm,and is capable of pumping adequate blood throughout a circulatorysystem. This intrinsic rhythm is a function of intrinsic signalsgenerated by the sinoatrial node, or SA node, located in the upper rightatrium. The SA node periodically depolarizes, which in turn causes theatrial heart tissue to depolarize such that right and left atriacontract as the depolarization travels through the atrial heart tissue.The atrial depolarization signal is also received by theatrioventricular node, or AV node, which, in turn, triggers a subsequentventricular depolarization signal that travels through and depolarizesthe ventricular heart tissue causing the right and left ventricles tocontract.

Some patients, however, have irregular cardiac rhythms, referred to ascardiac arrhythmias. Cardiac arrhythmias result in diminished bloodcirculation because of diminished cardiac output. Atrial fibrillation isa common cardiac arrhythmia that reduces the pumping efficiency of theheart. Atrial fibrillation is characterized by rapid, irregular,uncoordinated depolarizations of the atria. These depolarizations maynot originate from the SA node, but may instead originate from anarrhythmogenic substrate, such as an ectopic focus, within the atrialheart tissue. The reduced pumping efficiency due to atrial fibrillationrequires the ventricles to work harder, which is particularlyundesirable in sick patients that cannot tolerate additional stresses.As a result of atrial fibrillation, patients must typically limitactivity and exercise.

An even more serious problem, however, is the induction of rapid andirregular ventricular heart rhythms by the atrial fibrillation.Irregular atrial depolarization signals associated with atrialfibrillation are received by the AV node and may be conducted to theventricles. During atrial fibrillation, the intervals betweenventricular depolarizations may be shortened and vary substantially.Such induced arrhythmias compromise pumping efficiency even moredrastically than atrial arrhythmias. This phenomenon is referred to asrapidly conducted atrial fibrillation, or “conducted AF.”

SUMMARY

This disclosure is directed toward delivering AV nodal stimulation todecrease the ventricular rate response to a conducted atrialtachyarrhythmia. AV nodal stimulation is employed during an atrialtachyarrhythmia episode with rapid ventricular conduction to distinguishventricular tachyarrhythmia from supraventricular tachycardia andthereby prevent delivering inappropriate therapy to a patient.

In one example, a method includes detecting an atrial tachyarrhythmia,during the detected atrial tachyarrhythmia, anticipating detection of aventricular tachyarrhythmia in a heart of a patient based on a thresholdvalue of a ventricular tachyarrhythmia event count, in response toanticipating detection of the ventricular tachyarrhythmia, deliveringelectrical stimulation to block the atrioventricular node of the heart,and terminating the delivery of electrical stimulation based on one ormore stimulation termination criteria.

In one example, a method includes anticipating detection of aventricular tachyarrhythmia in a heart of a patient and, in response toanticipating detection of the ventricular tachyarrhythmia, deliveringelectrical stimulation to block the atrioventricular node of the heartover an electrical stimulation delivery time period. The electricalstimulation is delivered based on a sense time period over whichventricular depolarizations can be sensed during the electricalstimulation delivery time period.

In another example, a system includes a stimulation generator and aprocessor. The stimulation generator is configured to deliver vagalstimulation to a patient. The processor is configured to anticipatedetection of a ventricular tachyarrhythmia in a heart of the patientand, in response to anticipating detection of the ventriculartachyarrhythmia, control the stimulation generator to deliver electricalstimulation to block the atrioventricular node of the heart over anelectrical stimulation delivery time period. The processor controls thestimulation generator to deliver the electrical stimulation based on asense time period over which ventricular depolarizations can be sensedduring the electrical stimulation delivery time period.

In another example, a computer-readable storage medium includesinstructions for causing a programmable processor to anticipatedetection of a ventricular tachyarrhythmia in a heart of a patient and,in response to anticipating detection of the ventriculartachyarrhythmia, deliver electrical stimulation to block theatrioventricular node of the heart over an electrical stimulationdelivery time period. The electrical stimulation is delivered based on asense time period over which ventricular depolarizations can be sensedduring the electrical stimulation delivery time period.

In another example, a system includes means for anticipating detectionof a ventricular tachyarrhythmia in a heart of a patient and means for,in response to anticipating detection of the ventriculartachyarrhythmia, delivering electrical stimulation to block theatrioventricular node of the heart over an electrical stimulationdelivery time period. The electrical stimulation is delivered based on asense time period over which ventricular depolarizations can be sensedduring the electrical stimulation delivery time period.

In another example, a method includes anticipating a ventriculartachyarrhythmia in a heart of a patient and, in response to anticipatingthe ventricular tachyarrhythmia, delivering electrical stimulation toblock the atrioventricular node of the heart over an electricalstimulation delivery time period. The electrical stimulation isdelivered based on a sense time period over which ventriculardepolarizations can be sensed during the electrical stimulation deliverytime period.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy systemcomprising an implantable medical device (IMD) that may be used tomonitor one or more physiological parameters of a patient and/or providetherapy to the heart of a patient.

FIG. 2 is a conceptual diagram further illustrating the IMD and leads ofthe system of FIG. 1 in conjunction with the heart.

FIG. 3 is a conceptual diagram illustrating an example implantationlocation for a right atrial lead through which AV nodal vagalstimulation may be delivered.

FIG. 4 is a functional block diagram illustrating an exampleconfiguration of an IMD.

FIG. 5 is block diagram of an example external programmer thatfacilitates user communication with the IMD.

FIG. 6 is a block diagram illustrating an example system that includesan external device, such as a server, and one or more computing devicesthat are coupled to the IMD and programmer shown in FIG. 1 via anetwork.

FIG. 7 is a flow diagram of an example method of delivering vagalstimulation to a patient.

FIG. 8 is a flow diagram of one example of details of the method ofdelivering vagal stimulation to a patient illustrated in FIG. 7 .

FIG. 9 is a graph illustrating a series of vagal stimulation burstsdelivered over the course of a number of depolarizations andrepolarizations of the heart of a patient.

DETAILED DESCRIPTION

This disclosure is directed toward delivering electrical stimulation,e.g., vagal stimulation, to regulate the atrioventricular node (AV node)of the heart of a patient. The electrical stimulation may blockconduction of depolarizations to the ventricles via the AV node, but, ingeneral, may include any stimulation that modifies conduction of the AVnode. Vagal stimulation, for example, may regulate the cardiac autonomicnervous system by increasing parasympathetic activity in order to reducethe ventricular rate response to a conducted atrial tachyarrhythmia byblocking atrial signals from propagating to the ventricles through theAV node. The electrical stimulation may be employed during an atrialtachyarrhythmia episode with rapid ventricular conduction to distinguishventricular tachyarrhythmia from supraventricular tachycardia andprevent delivering inappropriate therapy to a patient, e.g., deliveringa high voltage shock in response to an incorrectly diagnosed ventriculartachyarrhythmia. An atrial tachyarrhythmia includes, e.g., atrialfibrillation and atrial tachycardia. Similarly, a ventriculartachyarrhythmia includes, e.g., ventricular fibrillation and ventriculartachycardia.

Due to the close relation of vagal innervation to the AV node, highfrequency stimulation, e.g., in the form of bursts of pulses or acontinuous train of pulses, of the AV node and/or neural fibersproximate to the AV node may provide vagal stimulation appropriate forthe examples described herein. Hereinafter, vagal stimulation will beprimarily described with respect to the example of AV nodal stimulation.However, in other examples, vagal stimulation may be delivered at otherlocations including, e.g., epicardially at one or more fat pads, ordirectly to the vagus nerve via, for example, a cuff electrode.Additionally, as noted above, although vagal stimulation is described insome examples below as the mechanism for regulating conduction of the AVnode, in other examples according to this disclosure, other neurologicalstructures that directly or indirectly innervate the AV node may bestimulated.

Under certain circumstances, medical devices that treat cardiacarrhythmias may incorrectly diagnose the arrhythmia and deliver aninappropriate therapy to a patient in response thereto. In one example,an incorrect diagnosis and inappropriate therapy delivery may arise whena medical device misinterprets an atrial tachyarrhythmia with rapidventricular conduction as a more serious ventricular tachyarrhythmia.The medical device detects the rapid rate response of the ventricleduring the atrial tachyarrhythmia and interprets the increasedcontraction rate as a ventricular tachyarrhythmia instead of asupraventricular tachycardia. In response to the incorrectly detectedventricular tachyarrhythmia, the device delivers an inappropriatetherapy to the patient in the form of, e.g., a high voltage shock.

Techniques are disclosed herein that distinguish between ventriculartachyarrhythmia and supraventricular tachycardia, e.g. atrialtachyarrhythmia with rapid ventricular rate response by deliveringelectrical stimulation, e.g. vagal stimulation, to block the AV node andthereby slow the ventricular rate to unmask the detected ventriculartachyarrhythmia as a supraventricular tachycardia. Additionally, if theAV-node block is not effective in slowing the ventricular rate, then aventricular tachyarrhythmia episode may be occurring and the device maydeliver appropriate therapy, e.g. a high voltage shock to the patient totreat the arrhythmia.

FIG. 1 is a conceptual diagram illustrating an example system 10 thatmay be used to monitor one or more physiological parameters of patient14 and/or to provide therapy to heart 12 of patient 14. System 10includes an implantable medical device (IMD) 16, which is coupled toleads 18, 20, and 22, and programmer 24. IMD 16 may be an implantablepacemaker that provides electrical signals to heart 12 via electrodescoupled to one or more of leads 18, 20, and 22. In addition to pacingtherapy, IMD 16 may deliver AV nodal stimulation and/or neurostimulationsignals. In some examples, IMD 16 may also include cardioversion and/ordefibrillation functionalities. Patient 14 is ordinarily, but notnecessarily, a human patient.

Leads 18, 20, 22 extend into the heart 12 of patient 14 to senseelectrical activity of and/or deliver electrical stimulation to theheart. In the example shown in FIG. 1 , right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), and right atrium 26, and into right ventricle 28. RV lead18 may be used to deliver RV pacing to heart 12. Left ventricular (LV)lead 20 extends through one or more veins, the vena cava, right atrium26, and into the coronary sinus 30 to a region adjacent to the free wallof left ventricle 32 of heart 12. LV lead 20 may be used to deliver LVpacing to heart 12.

In some examples, LV lead 20 may be used in combination with RV lead 18to deliver biventricular pacing to heart 12, which may provide cardiacresynchronization therapy (CRT) to heart 12. CRT may be used to treatheart failure-inducted conduction disturbances and/or ventriculardyssynchrony. In some cases, CRT may help restore the mechanicalsequence of ventricular activation and contraction. Additionally, CRTmay involve biventricular pacing, e.g., via RV lead 18 and LV lead 20,to synchronize the contraction of both ventricles. In other examples,CRT may involve pacing one of the ventricles, e.g., LV 32 via LV lead20, to synchronize its contraction with that of the other ventricle.

Right atrial (RA) lead 22 extends through one or more veins and the venacava, and into the right atrium 26 of heart 12. RA lead 22 may bepositioned in the inferior portion of right atrium 26. In some examples,RA lead 22 may be positioned in the posterior portion of right atrium 26around the coronary sinus ostium, such as posteriorly to the coronarysinus ostium, and along the septum that separates right atrium 26 andleft atrium 36. For example, RA lead 22 may be positioned such that RAlead 22 may sense electrical activity within right atrium 26, pace rightatrium 26, and also deliver a stimulation signal to or proximate to theAV node, e.g., to or proximate to the AV nodal vagal fat pad.

In some examples, system 10 may include an additional lead or leadsegment (not shown in FIG. 1 ) that deploys one or more electrodeswithin the vena cava or other vein, or within or near the aorta. Inother examples, system 10 may include one or more additional intravenousor extravascular leads or lead segments that deploy one or moreelectrodes epicardially, e.g., near an epicardial fat pad, or proximateto the vagal nerve. In other examples, system 10 need not include one ofventricular leads 18 and 20, such as where CRT is provided by pacing oneventricle, rather than both ventricles.

IMD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes (not shown in FIG. 1 ) coupledto at least one of the leads 18, 20, 22. In some examples, IMD 16provides pacing pulses to heart 12 based on the electrical signalssensed within heart 12. The configurations of electrodes used by IMD 16for sensing and pacing may be unipolar or bipolar.

IMD 16 may trigger ventricular pacing, e.g., RV, LV, or biventricularpacing, based on atrial depolarizations sensed via RA lead 22. Asanother example, RA lead 22 may deliver atrial pacing, and IMD 16 maytrigger ventricular pacing based on atrial-paced events. In someexamples, RV lead 18 and/or LV lead 20 may sense ventriculardepolarizations, and IMD 16 may trigger ventricular pacing, e.g., RV,LV, or biventricular pacing, based on whether RV lead 18 and/or LV lead20 detects an intrinsic ventricular depolarization within a defined timeinterval following the atrial sensed or paced event. The time intervalbetween an atrial sensed or paced event and delivery of a pacing pulseto one or more of the ventricles may be referred to as an AV interval.

IMD 16 may also provide neurostimulation therapy, defibrillation therapyand/or cardioversion therapy via electrodes located on at least one ofthe leads 18, 20, 22. IMD 16 may detect an arrhythmia of heart 12, suchas fibrillation of ventricles 28 and 32, and deliver defibrillationtherapy to heart 12 in the form of electrical shocks, which may take theform of pulses. In some examples, IMD 16 may be programmed to deliver aprogression of therapies, e.g., shocks with increasing energy levels,until a fibrillation of heart 12 is stopped. IMD 16 may detectfibrillation employing one or more appropriate fibrillation detectiontechniques. IMD 16 may similarly deliver anti-tachycardia pacing orcardioversion in response to detecting tachycardia of ventricles 28 and32.

The techniques disclosed herein are directed to employing electricalstimulation, e.g. vagal stimulation to block the AV node during anatrial tachyarrhythmia episode with rapid ventricular conduction todistinguish ventricular tachyarrhythmia from supraventriculartachycardia and prevent delivering inappropriate therapy to a patient,e.g., delivering a high voltage shock in response to an incorrectlydiagnosed ventricular tachyarrhythmia. For example, IMD 16 may alsodetect an atrial tachyarrhythmia, such as atrial fibrillation, anddeliver AV nodal vagal stimulation to reduce the ventricular rateresponse to the atrial tachyarrhythmia. In one example, IMD 16 monitorsheart 12 of patient 14 for a ventricular tachyarrhythmia using, e.g.,one or more electrodes connected to one or more of leads 18, 20 and 22according to any of a number of appropriate ventricular tachyarrhythmiadetection techniques.

In addition to monitoring heart 12 for a ventricular tachyarrhythmia,IMD 16 is configured to detect an atrial tachyarrhythmia employing,e.g., RA lead 22 positioned to sense electrical activity within rightatrium 26. In one example, as a threshold to detecting an arrhythmia inheart 12 of patient 14, IMD 16 analyzes the rhythm of heart 12 forindications of sinus tachycardia. Accelerated heart rates commonlyindicate conditions for which patient 14 may need therapy, such as anatrial or ventricular tachyarrhythmia. However, rapid heart rates arealso caused by normal physiological conditions including, e.g.,exercise, stress, and certain emotional responses. Analyzing the rhythmof heart 12 of patient 14 for indications of sinus tachycardia,therefore, provides a confirmation that a treatable arrhythmia versusnormal physiological response is occurring in the patient's heart. Inone example, IMD 16 may be programmed to determine if the R-R intervalsof heart 12 are approximately equal to the P-P intervals, and/or todetermine if the R-R intervals and/or P-R intervals are within a setphysiological limit for patient 14, all of which may be indicative of asinus tachycardia, rather than a ventricular tachyarrhythmia.

In some examples, atrial tachyarrhythmia is indicated by morecontractions in atria 26, 36 than in ventricles 28, 32 of heart 12. P-Pinterval is a measure of the length of the depolarization andrepolarization cycle of atria 26, 36. Similarly, R-R interval is ameasure of the length of the depolarization and repolarization cycle ofventricles 28, 32. As such, the contraction rate of atria 26, 36increases as the P-P interval decreases.

IMD 16 may therefore detect an atrial tachyarrhythmia by, e.g.,detecting a P-P interval in heart 12 of patient 14 that is less than apercentage threshold of an R-R interval of the heart. In some examples,IMD 16 monitors heart 12 and stores a number of P-P and R-R intervalsand compares a median P-P interval to a median R-R interval. Thepercentage threshold of the R-R interval may be based on empirical dataindicating at what differential between the number of atrial andventricular contractions is an atrial tachyarrhythmia indicated. In oneexample, IMD 16 detects an atrial tachyarrhythmia when the devicedetects a median P-P interval that is less than approximately 93.75% ofa median R-R interval.

As explained above, in the course of treating patient 14, IMD 16 mayincorrectly interpret characteristics of the rhythm of heart 12 thatindicate a supraventricular tachycardia as a more serious ventriculartachyarrhythmia, e.g. a potentially life-threatening ventricularfibrillation. One situation in which IMD 16 is susceptible to suchconfusion arises from rapid ventricular conduction through the AV nodeduring atrial tachyarrhythmia, e.g., atrial fibrillation. In suchcircumstances, IMD 16 may incorrectly diagnose the rapid contractionrate of ventricles 28, 32 as a ventricular versus supraventricularphenomenon and deliver an inappropriate stimulation therapy to patient14, such as delivering a high voltage shock to heart 12. In order tomitigate the risk of misdiagnosis and inappropriate therapy deliverybased thereon, therefore, IMD 16 may anticipate imminent ventriculartachyarrhythmia detection and take measures, described below, prior tothe detection.

In one example, IMD 16 monitors heart 12 for ventricular tachyarrhythmiaevents, e.g. a threshold ventricular contraction rate and increments acounter upon detection of each such event. IMD 16 may anticipateventricular tachyarrhythmia detection when the ventriculartachyarrhythmia event count exceeds a threshold value. By anticipatingpotentially incorrect ventricular tachyarrhythmia detection, IMD 16 isable to intervene with AV nodal stimulation, e.g. vagal stimulation toimpede rapid atrioventricular conduction during atrial tachyarrhythmiabefore the device incorrectly diagnoses and treats patient 14 for aventricular tachyarrhythmia. In some examples, IMD 16 need not intervenewith vagal or another type of electrical stimulation until an imminentdetection is indicated with a threshold confidence by basing theanticipation of a ventricular tachyarrhythmia on a number of eventsindicative of such a condition.

IMD 16 may take one more precautions as a precondition to delivering AVnodal stimulation to heart 12 to reduce the ventricular rate response toan atrial tachyarrhythmia. In some examples, IMD 16 may measure the R-Rinterval of heart 12 to confirm that the contraction rate of ventricles28, 32 is in a range appropriate for intervening with AV nodalstimulation. In particular, IMD 16 may determine a median R-R intervalfrom a number of measured R-R intervals for heart 12 to confirm that thecontraction rate of ventricles 28, 32 is below a maximum threshold andabove a minimum threshold. For example, a supraventricular tachycardiafor which AV nodal stimulation may be employed is not likely if theventricular contraction rate is too fast (i.e. RR median is too low),e.g. if the rate is greater than 240 beats per minute (bpm). Conversely,if the contraction rate of ventricles 28, 32 is too slow (i.e. R-Rmedian too high), the ventricular conduction through the AV node duringatrial tachyarrhythmia may not be considered rapid enough to evenwarrant attention, let alone intervention with AV nodal stimulation.

In one example, IMD 16 may deliver AV nodal stimulation to patient 14 inresponse to a command from a user, such as patient 14 or a clinicianvia, e.g. programmer 24. In such examples, user activation of AV nodalstimulation may only be allowed in the event IMD 16 detects atrial orventricular tachyarrhythmia episode in heart 12 of patient 14.

In some examples, in the event an atrial tachyarrhythmia is detected,and a ventricular tachyarrhythmia is not detected but is anticipated,e.g., based on detection of a number of short ventricular intervals, IMD16 may deliver AV nodal stimulation to patient 14. In one example, IMD16 employs one or more electrodes of RA lead 22 to deliver stimulationto or proximate to the AV node, e.g., to or proximate to the AV nodalvagal fat pad. IMD 16 delivers vagal stimulation in the form of burstsof pulses or a continuous train of pulses. The stimulation may bedelivered according to one or more programmed stimulation parametersincluding, e.g., amplitude, pulse width and frequency, as well as thenumber of pulses within a burst. For example, IMD 16 may deliver vagalstimulation via electrodes on lead 22 with a frequency in a range fromapproximately 20 Hz to approximately 100 Hz, and amplitude in a rangefrom approximately 0.5 volts to approximately 8 volts. IMD 16 deliversvagal stimulation to patient 14 to block the AV node of heart 12, whichmay act to reduce the ventricular conduction and contraction rate causedby a supraventricular tachycardia, e.g., during an atrial fibrillation.

IMD 16 may, in some examples, also be programmed with stimulationparameters configured to act as safety precautions to guard against AVnodal stimulation preventing appropriate sensing in ventricles 28, 32.For example, as IMD 16 delivers high frequency stimulation, e.g., in theform of bursts of pulses, blanking periods in which the device does notsense ventricular activity may accumulate. A blanking period is a timeperiod over which IMD 16 “blanks” amplifier(s) that are used for sensingto protect them from the high frequency energy being used to deliverstimulation. In this manner, the blanking period is generally equal tothe stimulation time period. In some cases, IMD 16 may be programmed toblank an amplifier for a time period that is nominally longer than thestimulation time period as an extra precaution.

There is a risk, as IMD 16 delivers high frequency stimulation, that thestimulation time periods, and therefore the blanking periods, reach athreshold level beyond which IMD 16 may not be able to detect thedevelopment of serious arrhythmias in ventricles 28, 32, e.g.ventricular fibrillation. The stimulation frequency may therefore bebounded by limits that are designed to prevent the accumulation ofblanking periods beyond a threshold level. For example, a period of timeover which a stimulation burst is delivered by IMD 16 may be based onthe contraction frequency in ventricles 28, 32 such that the burstperiod does not subsume the period between ventricular contractions. Inthis manner, IMD 16 may base the stimulation burst period on, e.g., amedian R-R interval for heart 12. In one example, IMD 16 may limit thestimulation burst period to approximately 50% of a median R-R intervalfor heart 12 of patient 14 such that between ventricular contractions50% of the time the stimulation burst is delivered and 50% of the timeis retained for sensing the activity of ventricles 28, 32.

In some examples, IMD 16 may also be configured to synchronize thedelivery of AV nodal stimulation with a QRS complex of heart 12. Inparticular, IMD 16 may be configured to deliver, e.g. AV nodal vagalstimulation in a refractory period between depolarization/repolarizationcycles. During the refractory period, the stimulation is less likely todepolarize heart 12, and, in particular, ventricles 28, 32. In oneexample, IMD 16 may also synchronize delivery of AV nodal stimulationwith a P-wave, which may act to reduce the likelihood of inducing anAT/AF episode after such an episode terminates.

IMD 16 continues to deliver AV nodal stimulation until one or morestimulation termination criteria are satisfied, at which point thedevice terminates the stimulation. In one example, the terminationcriteria includes at least one of expiration of a programmed AV nodalstimulation delivery time period, an accumulation of blanking timeperiods that exceeds a threshold percentage of a AV nodal stimulationdelivery time period, failure to detect a threshold ventricular rateresponse within a AV nodal stimulation response time period, ordetection of a ventricular tachyarrhythmia. IMD 16 may also terminatestimulation in the event normal conduction in atriums 26, 36 of heart 12is detected.

In some examples, IMD 16 is programmed, e.g. according to a therapyprogram, to deliver AV nodal stimulation for a specific period of time.The programmed stimulation time period may be set to a value thatprovides a sufficient amount of time for IMD 16 to test theeffectiveness of the stimulation in modulating the ventricular rateresponse. Additionally, regardless of other termination criteria, thestimulation time period may be set to a value that provides hysteresissuch that IMD 16 is not rapidly toggling between turning AV nodalstimulation on and off. In one example, IMD 16 is programmed with a AVnodal stimulation time period in a range from approximately 20 secondsto approximately 30 seconds.

As described above with reference to the parameters by which IMD 16delivers AV nodal stimulation to patient 14, there is a risk that, asIMD 16 delivers high frequency stimulation bursts, blanking periods willaccumulate beyond a threshold such that IMD 16 may not be able to detectventricular depolarizations associated with the development of aventricular tachyarrhythmia, e.g. ventricular fibrillation in heart 12of patient 14. IMD 16 may, therefore, determine an amount of time overwhich ventricular depolarizations need to be sensed during the deliveryof stimulation, and deliver the stimulation based on the determinedamount of time. In one example, IMD 16 may monitor delivery of vagalstimulation to block the AV node of heart 12 and the periods of time thedevice is sensing activity in ventricles 28, 32 during stimulation toensure that the stimulation burst period, which generally corresponds tothe blanking period, does not exceed a threshold percentage of theperiod between depolarizations of ventricles 28, 32 such that the burstperiod does not subsume the period between the ventricular contractions.In one example, the threshold percentage is 50% such that, for example,the stimulation burst period for vagal stimulation delivered by IMD 16does not exceed 50% of a median R-R interval for heart 12 of patient 14.

In one such example, IMD 16 may deliver multiple stimulation bursts ormultiple series of pulses for a total vagal stimulation time period of30 seconds. Over the 30 second vagal stimulation time period, a windowof at least 15 seconds in which IMD 16 may sense activity in ventricles28, 32 is needed. In the event the 50% threshold is exceeded, e.g. IMD16 only senses activity in heart 12 for 10 seconds of a 30 secondperiod, IMD 16 may terminate delivery of vagal stimulation to patient14.

In some examples, IMD 16 is programmed to allow the stimulation burstperiod to exceed a threshold for a period at the beginning of thedelivery of AV nodal stimulation. As IMD 16 delivers AV nodalstimulation, e.g. vagal stimulation to heart 12, the contraction rate inventricles 28, 32 may begin to slow, which may, in turn, increase theR-R interval for heart 12. As the R-R interval increases, the percentageof the interval over which the stimulation is delivered decreases.

However, before the effect of the vagal stimulation is able to slow thecontractions of ventricles 28, 32, the stimulation burst period mayexceed the threshold. Therefore, without permitting the stimulationburst period to exceed the threshold for a brief period of time, thevagal stimulation delivered by IMD 16 may not have an adequateopportunity to affect the rapid ventricular contraction rate in heart12. In one example, IMD 16 is configured to permit the stimulation burstperiod to exceed a threshold percentage of the contraction frequency inventricles 28, 32 for a time period corresponding to a stimulationresponse time, as described below.

In addition to AV nodal stimulation time period expiration and blankingperiod accumulation, IMD 16 may be programmed to terminate the deliveryof AV nodal stimulation in the event a minimum ventricular rate responseis not observed within a programmed response time period. In oneexample, IMD 16 may be programmed to terminate, e.g. vagal stimulationif the contraction rate of ventricles 28, 32 does not decrease by athreshold amount within the stimulation response time period. Theminimum ventricular rate response may differ from one patient to anotherand may be set as a relative percentage reduction or as an absolutevalue rate reduction. In examples in which the minimum rate response isset as an absolute value, the value by which IMD 16 measures rateresponse may be tiered depending on the observed contraction rate ofventricles 28, 32, i.e. the minimum rate response may be higher forhigher ventricular contraction rates and lower for lower ventricularcontraction rates.

In one example, IMD 16 is programmed with a vagal stimulation responsetime period of approximately 10 seconds or in a range of 5 to 10 beatsof heart 12. Additionally, IMD 16 is programmed with a minimum rateresponse approximately equal to a 20% rate reduction from the initialcontraction rate of ventricles 28, 32. In another example, IMD 16 isprogrammed with a minimum rate response approximately equal to 40 bpmfor higher initial ventricular contraction rates on the order of 180 bpmor higher, and a minimum rate response approximately equal to 20 bpm forlower initial ventricular contraction rates on the order of 120 bpm orlower.

In some examples, the programmed response time period may depend onactivity of the sympathetic system of patient 14. The parasympatheticsystem response generally has a quick on and off set. The sympatheticsystem, on the other hand, has a slow on and off set. In case theparasympathetic system is stimulated, a decrease in sympatheticactivation may occur, which, if it does occur, will make the AV noderesponse stronger. However, the sympathetic influence may not be noticedimmediately. In case there is a strong background sympathetic activitycounteracting parasympathetic action, it may take longer for the effectof AV nodal stimulation to become noticeable than if no sympatheticbackground activity is present. This sympathetic activity backgroundlevel depends on individual patient conditions/characteristics, e.g.arousal levels, circadian rhythm, health, if the patient is tired,anaethesia, medication, and other factors. In one example, theprogrammed response time period may be set based on, e.g. an averageactivation of the sympathetic system of patient 14. In one example,average activation of the sympathetic system of patient 14 may bedetermined by analyzing the low and high frequency content of thefrequency spectrum of the R-R interval during a baseline period of time(e.g. during no AT/AF episodes).

Referring again to FIG. 1 , programmer 24 comprises a handheld computingdevice, computer workstation, or networked computing device. Programmer24 includes a user interface that presents information to and receivesinput from a user. It should be noted that the user may also interactwith programmer 24 remotely via a networked computing device.

A user, such as a physician, technician, surgeon, electrophysiologist,or other clinician, interacts with programmer 24 to communicate with IMD16. For example, the user may interact with programmer 24 to retrievephysiological or diagnostic information from IMD 16. A user may alsointeract with programmer 24 to program IMD 16, e.g., select values foroperational parameters of the IMD.

In one example, a user may retrieve information regarding the rhythm ofheart 12, trends therein over time, or arrhythmic episodes from IMD 16using programmer 24. As another example, the user may use programmer 24to retrieve information from IMD 16 regarding other sensed physiologicalparameters of patient 14 or information derived from sensedphysiological parameters, such as the ventricular rate response of heart12 during one or more atrial tachyarrhythmia episodes. As anotherexample, the user may use programmer 24 to retrieve information from IMD16 regarding the performance or integrity of IMD 16 or other componentsof system 10, such as leads 18, 20 and 22, or a power source of IMD 16.As another example, the user may interact with programmer 24 to program,e.g., select parameters for, therapies provided by IMD 16, such as AVnodal vagal stimulation and, optionally, cardioversion and/ordefibrillation. In one example, the user employs programmer 24 toprogram IMD 16 with one or more of a threshold ventriculartachyarrhythmia event count, vagal stimulation parameters, andstimulation termination criteria.

IMD 16 and programmer 24 may communicate via wireless communicationusing a number of appropriate techniques including, e.g., low frequencyor radiofrequency (RF) telemetry, but other techniques are alsocontemplated. In some examples, programmer 24 includes a programminghead that may be placed proximate to the patient's body near the IMD 16implant site in order to improve the quality or security ofcommunication between IMD 16 and programmer 24.

FIG. 2 is a conceptual diagram illustrating IMD 16 and leads 18, 20, 22of therapy system 10 in greater detail. Leads 18, 20, 22 may beelectrically coupled to a signal generator and a sensing module of IMD16 via connector block 34. In some examples, proximal ends of leads 18,20, 22 may include electrical contacts that electrically couple torespective electrical contacts within connector block 34 of IMD 16. Inaddition, in some examples, leads 18, 20, 22 may be mechanically coupledto connector block 34 with the aid of set screws, connection pins, snapconnectors, or another suitable mechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of concentric coiled conductors separated fromone another by tubular insulative sheaths. Bipolar electrodes 40 and 42are located adjacent to a distal end of lead 18 in right ventricle 28.In addition, bipolar electrodes 44 and 46 are located adjacent to adistal end of lead 20 in left ventricle 32 and bipolar electrodes 48 and50 are located adjacent to a distal end of lead 22 in right atrium 26.

Electrodes 40, 44, and 48 may take the form of ring electrodes, andelectrodes 42, 46, and 50 may take the form of extendable helix tipelectrodes mounted retractably within insulative electrode heads 52, 54,and 56, respectively. In some examples, one or more of electrodes 42,46, and 50 may take the form of pre-exposed helix tip electrodes. Inother examples, one or more of electrodes 42, 46, and 50 may take theform of small circular electrodes at the tip of a tined lead or otherfixation element. Leads 18, 20, 22 also include elongated electrodes 62,64, 66, respectively, which may take the form of a coil. Each of theelectrodes 40, 42, 44, 46, 48, 50, 62, 64, and 66 may be electricallycoupled to a respective one of the coiled conductors within the leadbody of its associated lead 18, 20, 22, and thereby coupled torespective ones of the electrical contacts on the proximal end of leads18, 20, 22.

Helix tip electrode 50, which may be extendable or pre-exposed, of RAlead 22 may be inserted into the tissue of right atrium 26 tosubstantially fix RA lead 22 within right atrium 26. For example, helixtip electrode 50 may be inserted into or proximate to the endocardium ofthe septum that separates right atrium 26 and left atrium 36 at aposterior portion of right atrium 26. As described previously, RA lead22 may be positioned such that RA lead 22 may sense electrical activitywithin right atrium 26, pace right atrium 26, and also deliver astimulation signal to (or proximate to) the AV node, e.g., to (orproximate to) the AV nodal vagal fat pad. Helix tip electrode 50 may aidin maintaining RA lead 50 in the appropriate position to provide suchfunctionality.

In some examples, as illustrated in FIG. 2 , IMD 16 includes one or morehousing electrodes, such as housing electrode 58, which may be formedintegrally with an outer surface of hermetically-sealed housing 60 ofIMD 16 or otherwise coupled to housing 60. In some examples, housingelectrode 58 is defined by an uninsulated portion of an outward facingportion of housing 60 of IMD 16. Other division between insulated anduninsulated portions of housing 60 may be employed to define two or morehousing electrodes. In some examples, housing electrode 58 comprisessubstantially all of housing 60. As described in further detail withreference to FIG. 3 , housing 60 may enclose a signal generator thatgenerates therapeutic stimulation, such as cardiac pacing pulses anddefibrillation shocks, as well as a sensing module for monitoring therhythm of heart 12.

IMD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes 40, 42, 44, 46, 48, 50, 58,62, 64, and 66. The electrical signals are conducted to IMD 16 from theelectrodes via the respective leads 18, 20, 22 or, in the case ofhousing electrode 58, a conductor coupled to housing electrode 58. IMD16 may sense such electrical signals via any bipolar combination ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, and 66. Furthermore, anyof the electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, and 66 may be usedfor unipolar sensing in combination with housing electrode 58.

In some examples, IMD 16 delivers pacing pulses via bipolar combinationsof electrodes 40, 42, 44, 46, 48 and 50 to produce depolarization ofcardiac tissue of heart 12. In some examples, IMD 16 delivers pacingpulses via any of electrodes 40, 42, 44, 46, 48 and 50 in combinationwith housing electrode 58 in a unipolar configuration. For example,electrodes 40, 42, and/or 58 may be used to deliver RV pacing to heart12. Additionally or alternatively, electrodes 44, 46, and/or 58 may beused to deliver LV pacing to heart 12, and electrodes 48, 50 and/or 58may be used to deliver RA pacing to heart 12. Furthermore, IMD 16 maydeliver defibrillation pulses to heart 12 via any combination ofelongated electrodes 62, 64, 66, and housing electrode 58. Electrodes58, 62, 64, 66 may also be used to deliver cardioversion pulses to heart12. Electrodes 62, 64, 66 may be fabricated from any suitableelectrically conductive material, such as, but not limited to, platinum,platinum alloy or other materials known to be usable in implantabledefibrillation electrodes.

In accordance with the techniques disclosed herein, IMD 16 may alsodeliver AV nodal stimulation to block the AV node and thereby slow rapidventricular conduction therethrough during an atrial tachyarrhythmiaepisode. In one example, IMD 16 delivers AV nodal stimulation to heart12 via electrodes 48, 50, and/or 66 of RA lead 22, e.g., in a bipolarconfiguration or in a unipolar configuration in combination with housingelectrode 58. For example, IMD 16 may monitor heart 12 for a ventriculartachyarrhythmia, detect an atrial tachyarrhythmia, e.g., via anycombination of electrodes 48, 50, 56 and 58, anticipate a ventriculartachyarrhythmia detection, e.g. based on a threshold value of aventricular tachyarrhythmia event count, and deliver AV nodalstimulation to block the AV node of heart 12. The AV nodal stimulationmay reduce the ventricular rate response to the atrial tachyarrhythmia,which in turn will unmask a potentially misdiagnosed ventriculartachyarrhythmia and thereby prevent an inappropriate therapy from beingdelivered to patient 14. The AV nodal stimulation may include IMD 16determining an amount of time over which ventricular depolarizationsneed to be sensed during the delivery of stimulation and delivering thestimulation based on the determined amount of time.

The configuration of system 10 illustrated in FIGS. 1 and 2 is merelyone example. In other examples, a therapy system may include epicardialleads and/or patch electrodes instead of or in addition to thetransvenous leads 18, 20, 22 illustrated in FIG. 1 . Further, IMD 16need not be implanted within patient 14. In examples in which IMD 16 isnot implanted in patient 14, IMD 16 may deliver defibrillation pulsesand other therapies, as well as AV nodal stimulation to heart 12 viapercutaneous leads that extend through the skin of patient 14 to avariety of positions within or outside of heart 12.

In addition, in other examples, a system may include any suitable numberof leads coupled to IMD 16, and each of the leads may extend to anylocation within or proximate to heart 12. For example, other examples ofsystems may include three transvenous leads located as illustrated inFIGS. 1 and 2 , and an additional lead located within or proximate toleft atrium 36. As another example, other examples of therapy systemsmay include a single lead that extends from IMD 16 into right atrium 26or right ventricle 28, or two leads that extend into a respective one ofthe right ventricle 26 and right atrium 26.

FIG. 3 is a conceptual diagram illustrating an example implantationlocation for RA lead 22, through which AV nodal stimulation may bedelivered. FIG. 3 illustrates heart 12 with right atrium 26 exposed bydissection and retraction of outer wall of the right atrium. Althoughnot illustrated in FIG. 3 , the distal portion of RA lead 22 willgenerally be advanced to its implantation location within right atrium26 intravenously and through superior vena cava 70 (or, in some cases,inferior vena cava 72). In the illustrated example, the distal portionof RA lead 22 is positioned and implanted in the posterior portion ofright atrium 26, posterior to the coronary sinus ostium 74 and along theseptum 76 that separates right atrium 26 and left atrium 36. In oneexample, the distal portion of RA lead 22 may be positioned andimplanted inferior to the coronary sinus ostium 74. Helical tipelectrode 50 may engage the endocardial tissue to fix the distal portionlead 22 at this position.

FIG. 4 is a functional block diagram illustrating one exampleconfiguration of IMD 16 including processor 80, memory 82, signalgenerator 84, electrical sensing module 86, sensor 87, telemetry module88, and power source 98. Memory 82 may include computer-readableinstructions that, when executed by processor 80, cause IMD 16 andprocessor 80 to perform various functions attributed to IMD 16 andprocessor 80 herein. Memory 82 may include any volatile, non-volatile,magnetic, optical, or electrical media, such as a random access memory(RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital media.

Processor 80 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 80 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processor 80herein may be embodied as software, firmware, hardware or anycombination thereof. Processor 80 controls signal generator 84 todeliver stimulation therapy to heart 12 according to operationalparameters or programs, which may be stored in memory 82.

Signal generator 84 is electrically coupled to electrodes 40, 42, 44,46, 48, 50, 58, 62, 64, and 66, e.g., via conductors of the respectivelead 18, 20, 22, or, in the case of housing electrode 58, via anelectrical conductor disposed within housing 60 of IMD 16. Signalgenerator 84 is configured to generate and deliver electricalstimulation therapy to heart 12. For example, signal generator 84 maydeliver defibrillation shocks to heart 12 via at least two electrodes58, 62, 64, 66. Signal generator 84 may deliver pacing pulses via ringelectrodes 40, 44, 48 coupled to leads 18, 20, and 22, respectively,and/or helical electrodes 42, 46, and 50 of leads 18, 20, and 22,respectively. Signal generator 84 may also deliver AV nodal stimulationvia electrodes 48, 50, and/or 66 of RA lead 22, e.g., in a bipolarconfiguration or in a unipolar configuration in combination with housingelectrode 58. In some examples, signal generator 84 delivers one or moreof these types of stimulation in the form of electrical pulses. In otherexamples, signal generator 84 may deliver one or more of these types ofstimulation in the form of other signals, such as sine waves, squarewaves, or other substantially continuous time signals.

In some examples, signal generator 84 is configured to deliver AV nodalstimulation in the form of a series of high frequency pulses. Forexample, signal generator 84 may deliver AV nodal stimulation, e.g., viaelectrodes 48, 50, and/or 66 of RA lead 22, in a burst patterncharacterized by a plurality of pulse trains of high frequency pulses.This burst pattern may be particularly effective in interrupting theconduction of cardiac impulses across the AV node to reduce theventricular rate response during an atrial tachyarrhythmia, e.g. atrialfibrillation.

In some examples, the effect of AV nodal stimulation delivered topatient 14 may take a period of time to subside after signal generatorstops delivering the stimulation. As such, in one example, signalgenerator 84 may be configured to deliver AV nodal stimulation, stopdelivering stimulation for a period of time, e.g. one or moredepolarization/repolarization cycles of heart 12, and then begindelivering AV nodal stimulation again.

Memory 82 may store values for stimulation parameters that processor 80accesses to control delivery of AV nodal stimulation by signal generator84. Such stimulation parameters may include pulse duration, pulse trainduration, the number of pulses in a pulse train, pulse amplitude, pulsefrequency, and pulse train frequency. As one example, signal generator82 may control stimulation using a pulse duration of approximately 0.2milliseconds, a pulse train duration of approximately 250 milliseconds,an amplitude of approximately 4 volts, a pulse frequency ofapproximately 50 hertz, and a pulse train frequency of approximately 80pulse trains per minute. These values merely are examples and othervalues are also contemplated.

In some examples, memory 82 may store other operation parameters of IMD16 by which the device delivers stimulation to patient 12 including,e.g., threshold atrial tachyarrhythmia detection criteria, ventriculartachyarrhythmia event count, and stimulation termination criteria. Forexample, memory 82 may store values of parameters related to as well asinstructions for detecting indications of sinus tachycardia as athreshold to detecting an atrial tachyarrhythmia. As another example,memory 82 store particular atrial and ventricular intervals or groups ofintervals employed by processor 80 of IMD 16 to detect an atrialtachyarrhythmia. For example, memory 82 may store a median P-P intervaland/or a median R-R interval for heart 12 of patient 14. In anotherexample, memory 82 stores one or more termination criteria employed byIMD 16 to determine when to terminate delivering AV nodal stimulation topatient 14.

In some examples, memory 82 may also store suitable ranges for one ormore stimulation parameters. As one example, memory 82 stores a pulsefrequency range of approximately 20 Hz to approximately 100 Hz. In otherexamples, the pulse frequency may fall outside of this range. In anotherexample, memory 82 stores an amplitude range of approximately 0.5 voltsto approximately 8 volts.

In one example, signal generator 84 may be configured to deliverstimulation adaptively to patient 14 according to parameters that aredetermined based on the effect of past stimulation. In one example,processor 80 may control stimulation generator 84 to deliver stimulationto patient 14 including one or more of frequency, pulse width, andamplitude values set based on a measured effect of past stimulation, orother physiological responses or parameters of the patient. Adaptivestimulation may save energy and extend the longevity of IMD 16.

Signal generator 84 may include a switch module and processor 80 may usethe switch module to select, e.g., via a data/address bus, which of theavailable electrodes are used to deliver stimulation signals, e.g.,defibrillation, pacing, and/or AV nodal stimulation signals. The switchmodule may include a switch array, switch matrix, multiplexer, or anyother type of switching device suitable to selectively couple a signalto selected electrodes.

Electrical sensing module 86 monitors signals from at least one ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64 or 66 in order to monitorelectrical activity of heart 12. Electrical sensing module 86 may alsoinclude a switch module to select which of the available electrodes areused to sense the heart activity. In some examples, processor 80 mayselect the electrodes that function as sense electrodes, or the sensingconfiguration, via the switch module within electrical sensing module86, e.g., by providing signals via a data/address bus.

In some examples, electrical sensing module 86 includes multipledetection channels, each of which may comprise an amplifier. Eachsensing channel may detect electrical activity in respective chamber ofheart 12, and may be configured to detect either R-waves or P-waves. Insome examples, electrical sensing module 86 or processor 80 may includean analog-to-digital converter for digitizing the signal received from asensing channel for electrogram signal processing by processor 80. Inresponse to the signals from processor 80, the switch module withinelectrical sensing module 86 may couple the outputs from the selectedelectrodes to one of the detection channels or the analog-to-digitalconverter.

During pacing, escape interval counters maintained by processor 80 maybe reset upon sensing of R-waves and P-waves with respective detectionchannels of electrical sensing module 86. Signal generator 84 mayinclude pacer output circuits that are coupled, e.g., selectively by aswitching module, to any combination of electrodes 40, 42, 44, 46, 48,50, 58, 62, or 66 appropriate for delivery of a bipolar or unipolarpacing pulse to one or more of the chambers of heart 12. Processor 80may control signal generator 84 to deliver a pacing pulse to a chamberupon expiration of an escape interval. Processor 80 may reset the escapeinterval counters upon the generation of pacing pulses by stimulationgenerator 84, or detection of an intrinsic depolarization in a chamber,and thereby control the basic timing of cardiac pacing functions. Theescape interval counters may include P-P, V-V, RV-LV, A-V, A-RV, or A-LVinterval counters, as examples. The value of the count present in theescape interval counters when reset by sensed R-waves and P-waves may beused by processor 80 to measure the durations of R-R intervals, P-Pintervals, P-R intervals and R-P intervals. Processor 80 may use thecount in the interval counters to detect a tachyarrhythmia event, suchas an atrial or ventricular fibrillation or tachycardia and/or detect aheart rate, such as an atrial rate or ventricular rate. For example,processor 80 may measure and record a number of P-P intervals and R-Rintervals for heart 12, from which the processor may detect an atrialtachyarrhythmia by detecting a median P-P interval in heart 12 ofpatient 14 that is less than a percentage threshold of a median R-Rinterval of the heart.

Processor 80 may also derive other physiological parameters from signalssensed via electrical sensing module 86. For example, processor 80 mayestablish one or more indicators of ejection fraction and/or heartfailure status from electrical signals sensed via electrical sensingmodule 86. In particular, impedance signals may be used to determineflow or pressure, which may indicate ejection fraction and/or heartfailure status.

In one example, processor 80, in conjunction with memory 82, signalgenerator 84, and sensing module 86 detects an atrial tachyarrhythmia,such as atrial fibrillation, and delivers AV nodal stimulation to reducethe ventricular rate response to the atrial tachyarrhythmia. Forexample, processor 80 monitors heart 12 of patient 14 for a ventriculartachyarrhythmia by controlling sensing module 86 to configure one ormore electrodes connected to one or more of leads 18, 20 and 22 as senseelectrodes and by employing any of a number of appropriate ventriculartachyarrhythmia detection techniques.

In addition to monitoring heart 12 for a ventricular tachyarrhythmia,processor 80 may be configured to detect an atrial tachyarrhythmia bycontrolling sensing module 86 to employ, e.g., RA lead 22 positioned tosense electrical activity within right atrium 26. In one example, as athreshold to detecting an arrhythmia in heart 12 of patient 14,processor 80 of IMD 16 analyzes the rhythm of heart 12 for indicationsof sinus tachycardia. Accelerated heart rates, such as an atrial orventricular tachyarrhythmia, commonly indicate conditions for whichpatient 14 may need therapy. However, rapid heart rates are also causedby normal physiological conditions including, e.g., exercise, stress,and certain emotional responses. Analyzing the rhythm of heart 12 ofpatient 14 for indications of sinus tachycardia, therefore, provides aconfirmation that a treatable arrhythmia versus normal physiologicalresponse is occurring in the patient's heart.

In one example, processor 80 is programmed to control sensing module 86to sense cardiac electrograms (EGMs) of heart 12, from which processor80 determines if the R-R interval of heart 12 is approximately equal tothe P-P interval. A one-to-one ratio between R-R and P-P intervalsindicates balanced atrial and ventricular contraction rates, which maybe indicative of a sinus tachycardia versus a tachyarrhythmia. Inanother example, processor 80 determines if R-R interval and/or the P-Rinterval are within a set physiological limit for patient 14, which mayalso be indicative of a sinus tachycardia versus a tachyarrhythmia.

In addition to checking for indications of sinus tachycardia, processor80 may also control sensing module 86 to monitor EGMs from heart 12 tomeasure one or more P-P and R-R intervals. In some examples, atrialtachyarrhythmia is indicated by more contractions in atria 26, 36 thanin ventricles 28, 32 of heart 12. P-P interval is a measure of thelength of the depolarization and repolarization cycle of atria 26, 36.Similarly, R-R interval is a measure of the length of the depolarizationand repolarization cycle of ventricles 28, 32. As such, the contractionrate of atria 26, 36 increases, as the P-P interval decreases. Processor80 may therefore employ sensing module 86 to detect an atrialtachyarrhythmia by, e.g., detecting a P-P interval in heart 12 ofpatient 14 that is less than a percentage threshold of an R-R intervalof the heart. In some examples, processor 80 controls sensing module 86to monitor heart 12 and stores a number of P-P and R-R intervals onmemory 82. Processor 80 calculates a median P-P and R-R interval fromthe stored values on memory 82. Processor 80 then compares a median P-Pinterval to a median R-R interval. The percentage threshold of the R-Rinterval may be based on empirical data indicating at what differentialbetween the number of atrial and ventricular contractions is an atrialtachyarrhythmia indicated. In one example, processor 80 detects anatrial tachyarrhythmia when the device detects a median P-P intervalthat is less than approximately 93.75% of a median R-R interval.

In order to mitigate the risk of misdiagnosis and inappropriate therapydelivery based thereon, processor 80 of IMD 16 may be programmed toanticipate imminent ventricular tachyarrhythmia detection and takemeasures prior to such detection. In one example, processor 80 controlssensing module 86 to monitor heart 12 for ventricular tachyarrhythmiaevents, e.g., intervals between consecutive ventricular depolarizationsthat are less than a tachyarrhthmia threshold. Processor 80 increments acounter stored on memory 82 upon detection of each such event by sensingmodule 86. Processor 80 may anticipate a ventricular tachyarrhythmiadetection when the ventricular tachyarrhythmia event count stored inmemory 82 exceeds a threshold value, also stored, e.g., in memory 82. Byanticipating a potentially incorrect ventricular tachyarrhythmiadetection, processor 80 is able to intervene with AV nodal stimulationto reveal rapid ventricular conduction during atrial tachyarrhythmiamasked as ventricular tachyarrhythmia before the device incorrectlydiagnoses and treats patient 14. Conversely, processor 80 need notintervene with stimulation until an imminent detection is indicated witha threshold confidence by basing the anticipation of a ventriculartachyarrhythmia on a number of events indicative of such a condition.

IMD 16 may take one more precautions as a precondition to delivering AVnodal stimulation to heart 12 to reduce the ventricular rate response toan atrial tachyarrhythmia. In some examples, processor 80 of IMD 16 maycontrol sensing module 86 to measure the R-R interval of heart 12 toconfirm that the contraction rate of ventricles 28, 32 is in a rangeappropriate for intervening with AV nodal stimulation. In particular,processor 80 may control sensing module 86 to measure a number of R-Rintervals of heart 12, from which processor 80 determines a median R-Rinterval to confirm that the contraction rate of ventricles 28, 32 isbelow a maximum threshold and above a minimum threshold. For example, asupraventricular tachycardia for which AV nodal stimulation may beemployed is not likely if the ventricular contraction rate is too fast(i.e. RR median is too low), e.g. if the rate is greater than 240 beatsper minute (bpm). Conversely, if the contraction rate of ventricles 28,32 is too slow (i.e. R-R median too high), the ventricular conductionthrough the AV node during atrial tachyarrhythmia is likely not rapidenough to even warrant attention, let alone intervention withstimulation. Therefore, processor 80 may not control signal generator 84to deliver AV nodal stimulation, unless the median R-R interval is belowthe maximum threshold and above the minimum threshold.

In some examples, in the event an atrial tachyarrhythmia is detected,and a ventricular tachyarrhythmia is not detected but a ventriculartachyarrhythmia detection is anticipated, processor 80 may controlsignal generator 84 to deliver AV nodal stimulation to patient 14. Inone example, processor 80 controls signal generator 84 to employ one ormore electrodes of RA lead 22 to deliver stimulation to or proximate tothe AV node, e.g., to or proximate to the AV nodal fat pad. Signalgenerator 84 may deliver such stimulation in the form of bursts ofpulses or a continuous train of pulses. The stimulation may be deliveredby signal generator 84 according to one or more programmed stimulationparameters stored in memory 82 including, e.g., amplitude, pulse widthand frequency, as well as the number of pulses within a burst. Forexample, signal generator 84 may deliver AV nodal stimulation viaelectrodes on lead 22 with a frequency in a range from approximately 20Hz to approximately 100 Hz, and amplitude in a range from approximately0.5 volts to approximately 8 volts. Processor 80 controls signalgenerator to deliver electrical stimulation to patient 14 to block theAV node of heart 12, which may act to reduce ventricular conduction andcontraction rate caused by a supraventricular tachycardia, e.g., duringan atrial fibrillation.

IMD 16 may, in some examples, also be programmed with stimulationparameters configured to act as safety precautions to guard againstvagal stimulation preventing appropriate sensing in ventricles 28, 32.As signal generator 84 delivers high frequency stimulation, e.g., in theform of bursts of pulses, blanking periods in which the device does notsense ventricular activity accumulate. There is a risk, as IMD 16delivers high frequency stimulation, that the blanking periods, andtherefore the stimulation burst time periods reach a threshold levelbeyond which IMD 16 may not be able to detect the development of seriousarrhythmias in ventricles 28, 32, e.g. ventricular fibrillation. Thestimulation frequency may therefore be bounded by limits that aredesigned to prevent the accumulation of blanking periods beyond athreshold level. For example, a period of time over which a stimulationburst is delivered by signal generator 84 may be based on thecontraction frequency in ventricles 28, 32 such that the burst perioddoes not subsume the period between ventricular contractions. In thismanner, processor 80 of IMD 16 may base the stimulation burst periodover which a burst of stimulation is delivered by signal generator 84on, e.g., a median R-R interval for heart 12. In one example, processor80 may control signal generator 84 to deliver a stimulation burst for aperiod of time based on, e.g., a median R-R interval for heart 12 ascalculated by processor 80 from a number of R-R intervals measured bysensing module 86. In one example, processor 80 may limit thestimulation burst period to approximately 50% of a median R-R intervalfor heart 12 of patient 14 such that 50% of the time between ventricularcontractions the stimulation burst is delivered and 50% of the time isretained for sensing the activity of ventricles 28, 32.

In other examples, IMD 16 may also be configured to synchronize thedelivery of AV nodal stimulation with a QRS complex of heart 12. Inparticular, IMD 16 may be configured to deliver, e.g. AV nodal vagalstimulation in a refractory period between depolarization/repolarizationcycles. During the refractory period, the stimulation is less likely todepolarize heart 12, and, in particular, ventricles 28, 32.

IMD 16 continues to deliver AV nodal stimulation until one or morestimulation termination criteria are satisfied, at which point thedevice terminates the stimulation. In one example, the terminationcriteria includes at least one of expiration of a programmed AV nodalstimulation delivery time period, an accumulation of blanking timeperiods that exceeds a threshold percentage of a AV nodal stimulationdelivery time period, failure to detect a threshold ventricular rateresponse within a AV nodal stimulation response time period, ordetection of a ventricular tachyarrhythmia.

In some examples, IMD 16 is programmed, e.g. according to a therapyprogram stored in memory 82 to deliver AV nodal stimulation for aspecific period of time. The programmed stimulation time period may beset to a value that provides a sufficient amount of time for IMD 16 totest the effectiveness of the stimulation in modulating the ventricularrate response. Additionally, regardless of other termination criteria,the stimulation time period may be set to a value that provideshysteresis such that IMD 16 is not rapidly toggling between turning AVnodal stimulation on and off. In one example, processor 80 of IMD 16 isprogrammed with a AV nodal stimulation time period stored in memory 82in a range from approximately 20 seconds to approximately 30 seconds.

As described above with reference to the parameters by which IMD 16delivers AV nodal stimulation to patient 14, there is a risk that, asIMD 16 delivers high frequency stimulation bursts, blanking periods willaccumulate beyond a threshold such that sensing module 86 may not beable to detect the development of a ventricular tachyarrhythmia, e.g.ventricular fibrillation in heart 12 of patient 14. Processor 80 may,therefore, be programmed to control sensing module 86 to monitor the AVnodal stimulation delivered by signal generator 84 and the periods oftime sensing module 86 is sensing activity in ventricles 28, 32 duringstimulation to ensure that an accumulated blanking period, i.e. anaccumulation of stimulation burst periods does not exceed a thresholdpercentage of the total time over which AV nodal stimulation isdelivered. In one example, the threshold percentage is 50% such that,for example, during 30 seconds of AV nodal stimulation a window of atleast 15 seconds in which sensing module 86 may sense activity inventricles 28, 32 is needed. In the event the 50% threshold is exceededby the accumulated blanking period, e.g. sensing module 86 only sensesactivity in heart 12 for 10 seconds of a 30 second period, processor 80may control signal generator 84 to terminate delivery of AV nodalstimulation to patient 14.

In another example, processor 80 controls sensing module 86 and signalgenerator 84 based on each stimulation burst or series of stimulationpulses such that the stimulation burst period does not exceed athreshold percentage of the period between ventricular depolarizations.In one example, processor 80 may limit the stimulation burst perioddelivered by signal generator 84 to approximately 50% of a median R-Rinterval for heart 12 of patient 14 such that 50% of the time betweenventricular contractions the stimulation burst is delivered and 50% ofthe time is retained for sensing the activity of ventricles 28, 32.

As explained above with reference to FIG. 1 , in some examples,processor 80 may be programmed to allow the stimulation burst period toexceed a threshold percentage of the depolarization frequency inventricles 28, 32 for a brief period at the beginning of the delivery ofAV nodal stimulation before the effect of the stimulation is able toslow the depolarizations of ventricles 28, 32.

In addition to stimulation time period expiration and blanking periodaccumulation, processor 80 may be programmed to control signal generator84 to terminate the delivery of AV nodal stimulation in the event aminimum ventricular rate response is not observed within a programmedresponse time period. In one example, processor 80 may be programmed toterminate AV nodal stimulation if the contraction rate of ventricles 28,32 does not decrease by a threshold amount within the stimulationresponse time period. The minimum ventricular rate response may differfrom one patient to another and may be set as a relative percentagereduction or as an absolute value rate reduction. In examples in whichthe minimum rate response is set as an absolute value, the value bywhich processor 80 measures rate response may be tiered depending on theobserved contraction rate of ventricles 28, 32, e.g. the minimum rateresponse may be higher for higher initial ventricular contraction ratesand lower for lower ventricular contraction rates.

In one example, processor 80 is programmed with a AV nodal stimulationresponse time period of approximately 10 seconds or in a range of 5 to10 beats of heart 12. Additionally, processor 80 is programmed with aminimum rate response approximately equal to a 20% rate reduction fromthe initial contraction rate of ventricles 28, 32 measured by sensingmodule 86. In another example, IMD 16 is programmed with a minimum rateresponse approximately equal to 40 bpm for higher initial ventricularcontraction rates on the order of 180 bpm or higher, and a minimum rateresponse approximately equal to 20 bpm for lower initial ventricularcontraction rates on the order of 120 bpm or lower. The stimulationresponse time period and minimum ventricular rate response values, whichare employed by processor 80 as basis for controlling signal generator84 to terminate stimulation, may, in some examples, be stored in memory82 of IMD 16.

IMD 16 may also include one or more sensors 87 separate from electrodes40, 42, 44, 46, 48, 50, 58, 62, 64 and 66. Via a signal generated bysensor 87, processor may monitor one or more physiological parametersindicative of cardiac contraction, autonomic tone, heart failure, and/orejection fraction. Examples of sensors 87 that may generate a signalindicative of cardiac contraction include a intracardiac orintravascular pressure sensor, an accelerometer or other sensor capableof detecting heart or blood sounds, vibrations, or motion, an optical orultrasonic sensor capable of detecting changes in flow associated withcardiac contractions, or an optical sensor capable of detecting oxygensaturation changes associated with cardiac contractions. Processor 80may detect cardiac contractions based on signals from one or moresensors 87, and detect arrhythmias based on the detected cardiaccontractions.

Telemetry module 88 includes any suitable hardware, firmware, softwareor any combination thereof for communicating with another device, suchas programmer 24 (FIG. 1 ). Under the control of processor 80, telemetrymodule 88 may receive downlink telemetry from and send uplink telemetryto programmer 24 with the aid of an antenna, which may be internaland/or external. Processor 80 may provide the data to be uplinked toprogrammer 24 and receive downlinked data from programmer 24 via anaddress/data bus. In some examples, telemetry module 88 may providereceived data to processor 80 via a multiplexer.

In some examples, processor 80 transmits indications of detected atrialtachyarrhythmias and the ventricular rate response therefrom viatelemetry module 88. Processor 80 may also transmit, via telemetrymodule 88, information regarding AV nodal stimulation delivered bysignal generator 84 and a response to AV nodal stimulation, e.g.,detected by electrical sensing module 86.

The various components of IMD 16 are coupled to power source 90, whichmay include a rechargeable or non-rechargeable battery. Anon-rechargeable battery may be capable of holding a charge for severalyears, while a rechargeable battery may be inductively charged from anexternal device, e.g., on a daily or weekly basis. In other examples,power source 90 may include a supercapacitor.

FIG. 5 is block diagram of an example configuration of programmer 24. Asshown in FIG. 5 , programmer 24 includes processor 140, memory 142, userinterface 144, telemetry module 146, and power source 148. Programmer 24may be a dedicated hardware device with dedicated software forprogramming of IMD 16. Alternatively, programmer 24 may be anoff-the-shelf computing device running an application that enablesprogrammer 24 to program IMD 16.

A user may use programmer 24 to select therapy programs (e.g., sets ofoperational parameters), generate new therapy programs, or modifytherapy programs for IMD 16. The clinician may interact with programmer24 via user interface 144 which may include a display to present agraphical user interface to a user, and a keypad or another mechanismfor receiving input from a user.

Processor 140 can take the form one or more microprocessors, DSPs,ASICs, FPGAs, programmable logic circuitry, or the like, and thefunctions attributed to processor 102 herein may be embodied ashardware, firmware, software or any combination thereof. Memory 142 maystore instructions that cause processor 140 to provide the functionalityascribed to programmer 24 herein, and information used by processor 140to provide the functionality ascribed to programmer 24 herein. Memory142 may include any fixed or removable magnetic, optical, or electricalmedia, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM,or the like. Memory 142 may also include a removable memory portion thatmay be used to provide memory updates or increases in memory capacities.A removable memory may also allow patient data to be easily transferredto another computing device, or to be removed before programmer 24 isused to program therapy for another patient. Memory 142 may also storeinformation that controls therapy delivery by IMD 16, such asstimulation parameter values associated with AV nodal vagal stimulationdelivered by signal generator 84 of IMD 16.

Programmer 24 may communicate wirelessly with IMD 16, such as using RFcommunication or proximal inductive interaction. This wirelesscommunication is possible through the use of telemetry module 146, whichmay be coupled to an internal antenna or an external antenna. Anexternal antenna that is coupled to programmer 24 may correspond to theprogramming head that may be placed over heart 12, as described abovewith reference to FIG. 1 . Telemetry module 146 may be similar totelemetry module 88 of IMD 16 (FIG. 3 ).

Telemetry module 146 may also be configured to communicate with anothercomputing device via wireless communication techniques, or directcommunication through a wired connection. Examples of local wirelesscommunication techniques that may be employed to facilitatecommunication between programmer 24 and another computing device includeRF communication according to the 802.11 or Bluetooth specificationsets, infrared communication, e.g., according to the IrDA standard, orother standard or proprietary telemetry protocols. In this manner, otherexternal devices may be capable of communicating with programmer 24without needing to establish a secure wireless connection. An additionalcomputing device in communication with programmer 24 may be a networkeddevice such as a server capable of processing information retrieved fromIMD 16.

In some examples, processor 140 may be configured to provide some or allof the functionality ascribed to processor 80 of IMD 16 herein. Forexample, processor 140 may receive indications of cardiacdepolarizations or contractions, a signal from one or more sensors 87,or information regarding detected atrial tachyarrhythmias and theventricular rate response to the detected arrhythmias from IMD 16 viatelemetry module 146. In some examples, processor 140 may initiate ormodify AV nodal stimulation by controlling signal generator 84 andsensing module 86 via telemetry modules 146, 88, as described hereinwith respect to IMD 16 and processor 80.

FIG. 6 is a block diagram illustrating an example system that includesan external device, such as a server 204, and one or more computingdevices 210A-210N, that are coupled to the IMD 16 and programmer 24shown in FIG. 1 via a network 202. In this example, IMD 16 may use itstelemetry module 88 to communicate with programmer 24 via a firstwireless connection, and to communication with an access point 200 via asecond wireless connection. In the example of FIG. 6 , access point 200,programmer 24, server 204, and computing devices 210A-210N areinterconnected, and able to communicate with each other, through network202. In some cases, one or more of access point 200, programmer 24,server 204, and computing devices 210A-210N may be coupled to network202 through one or more wireless connections. IMD 16, programmer 24,server 204, and computing devices 210A-210N may each comprise one ormore processors, such as one or more microprocessors, DSPs, ASICs,FPGAs, programmable logic circuitry, or the like, that may performvarious functions and operations, such as those described above withreference to processor 80 of IMD 16 and processor 140 of programmer 24.

Access point 200 may comprise a device that connects to network 202 viaany of a variety of connections, such as telephone dial-up, digitalsubscriber line (DSL), or cable modem connections. In other examples,access point 200 may be coupled to network 202 through different formsof connections, including wired or wireless connections. In someexamples, access point 200 may be co-located with patient 14 and maycomprise one or more programming units and/or computing devices (e.g.,one or more monitoring units) that may perform various functions andoperations described herein. For example, access point 200 may include ahome-monitoring unit that is co-located with patient 14 and that maymonitor the activity of IMD 16.

In some cases, server 204 may be configured to provide a secure storagesite for data that has been collected from IMD 16 and/or programmer 24.Network 202 may comprise a local area network, wide area network, orglobal network, such as the Internet. In some cases, programmer 24 orserver 206 may assemble data in web pages or other documents for viewingby trained professionals, such as clinicians, via viewing terminalsassociated with computing devices 210A-210N. The illustrated system ofFIG. 6 may be implemented, in some aspects, with general networktechnology and functionality similar to that provided by the MedtronicCareLink® Network developed by Medtronic, Inc., of Minneapolis, MN.

In some examples, processor 208 of server 204 may be configured toprovide some or all of the functionality ascribed to processor 80 of IMD16 herein. For example, processor 206 may receive indications of cardiacdepolarizations or contractions, a signal from one or more sensors 87,or information regarding detected atrial tachyarrhythmias and theventricular rate response to the detected arrhythmias from IMD 16 viaaccess point 200 or programmer 24 and network 202. Processor 206 mayalso initiate and/or terminate AV nodal stimulation delivered by signalgenerator 84 of IMD 16. In some examples, server 204 relays receivedindications of cardiac depolarizations or contractions, a signal fromone or more sensors 87, or information regarding detected atrialtachyarrhythmias and the ventricular rate response thereto provided byone or more of IMD 16 or programmer 24 to one or more of computingdevices 210 via network 202. A processor of a computing device 210 maysimilarly provide some or all of the functionality ascribed to processor80 of IMD 16 herein.

FIG. 7 is a flow diagram of an example method of delivering AV nodalstimulation to patient 14. The method of FIG. 7 includes monitoring arhythm of a heart of a patient (230), determining if a ventriculartachyarrhythmia is occurring in the patient's heart (232), detecting anatrial tachyarrhythmia (234), anticipating a ventricular tachyarrhythmiadetection based on a threshold value of a ventricular tachyarrhythmiaevent count (236), delivering AV nodal stimulation to block theatrioventricular node of the heart of the patient (238), determining ifone or more termination criteria have been satisfied (240), andterminating the delivery of AV nodal stimulation based on thestimulation termination criteria.

In one example, processor 80, in conjunction with memory 82, signalgenerator 84, and sensing module 86 detects an atrial tachyarrhythmia,such as atrial fibrillation, and delivers AV nodal vagal stimulation toreduce the ventricular rate response to the atrial tachyarrhythmia. Forexample, according to the method of FIG. 7 , processor 80 monitors heart12 of patient 14 (230) to determine if a ventricular tachyarrhythmia isoccurring (232) by controlling sensing module 86 to configure one ormore electrodes connected to one or more of leads 18, 20 and 22 as senseelectrodes and by employing any of a number of appropriate ventriculartachyarrhythmia detection techniques.

In addition to monitoring heart 12 for a ventricular tachyarrhythmia(230, 232), processor 80 may be configured to detect an atrialtachyarrhythmia (234) by controlling sensing module 86 to employ, e.g.,RA lead 22 positioned to sense electrical activity within right atrium26. In one example, as a threshold to detecting an arrhythmia in heart12 of patient 14 (234), processor 80 of IMD 16 analyzes the rhythm ofheart 12 for indications of sinus tachycardia. Accelerated heart ratescommonly indicate conditions for which patient 14 may need therapy, suchas an atrial or ventricular tachyarrhythmia. However, rapid heart ratesare also caused by normal physiological conditions including, e.g.,exercise, stress, and certain emotional responses. Analyzing the rhythmof heart 12 of patient 14 for indications of sinus tachycardia,therefore, provides a confirmation that a treatable arrhythmia versusnormal physiological response is occurring in the patient's heart.

In one example, processor 80 is programmed to control sensing module 86to sense cardiac EGMs of heart 12, from which processor 80 determines ifthe R-R interval of heart 12 is approximately equal to the P-P interval.A one-to-one ratio between R-R and P-P intervals indicates balancedatrial and ventricular contraction rates, which may be indicative of asinus tachycardia versus a tachyarrhythmia. In another example,processor 80 determines if R-R interval and/or the P-R interval arewithin a set physiological limit for patient 14, which may also beindicative of a sinus tachycardia versus a tachyarrhythmia.

In addition to checking for indications of sinus tachycardia, processor80 may also control sensing module 86 to monitor the ECG of heart 12 tomeasure one or more P-P and R-R intervals in order to detect an atrialtachyarrhythmia in the heart of patient 14 (234). In some examples,atrial tachyarrhythmia is indicated by more contractions in atria 26, 36than in ventricles 28, 32 of heart 12. P-P interval is a measure of thelength of the depolarization and repolarization cycle of atria 26, 36.Similarly, R-R interval is a measure of the length of the depolarizationand repolarization cycle of ventricles 28, 32. As such, the contractionrate of atria 26, 36 increases, as the P-P interval decreases. Processor80 may therefore employ sensing module 86 to detect an atrialtachyarrhythmia (234) by, e.g., detecting a P-P interval in heart 12 ofpatient 14 that is less than a percentage threshold of an R-R intervalof the heart. In some examples, processor 80 controls sensing module 86to monitor heart 12 and stores a number of P-P and R-R intervals onmemory 82. Processor 80 calculates a median P-P and R-R interval fromthe stored values on memory 82. Processor 80 then compares a median P-Pinterval to a median R-R interval. The percentage threshold of the R-Rinterval may be based on empirical data indicating at what differentialbetween the number of atrial and ventricular contractions is an atrialtachyarrhythmia indicated. In one example, processor 80 detects anatrial tachyarrhythmia (234) when sensing module 86 detects a median P-Pinterval that is less than approximately 93.75% of a median R-Rinterval.

In some cases, atrial tachyarrhythmia may be preceded by a disbalance inthe autonomic system of patient 14. The disbalance in the autonomicsystem of patient 14 may be detected by processor 80 controlling sensingmodule 86 to monitor the ECG of heart 12 to analyzing the frequencyspectrum of the rhythm of the heart, heart rate turbulence, or t-wavealternans. By detecting autonomic disbalance, in some examples, IMD 16may be able to detect an early warning signal of atrial tachyarrhythmiain patient 14 and begin delivering AV nodal stimulation before the onsetof the tachyarrhythmia episode.

In order to mitigate the risk of misdiagnosis and inappropriate therapydelivery based thereon, processor 80 of IMD 16 may be programmed toanticipate imminent ventricular tachyarrhythmia detection (236) and takemeasures prior to such detection. In one example, processor 80 controlssensing module 86 to monitor heart 12 for ventricular tachyarrhythmiaevents, e.g. a threshold ventricular contraction rate. Processor 80increments a counter stored on memory 82 upon detection of each suchevent by sensing module 86. Processor 80 may anticipate a ventriculartachyarrhythmia detection (236) when the ventricular tachyarrhythmiaevent count stored in memory 82 exceeds a threshold value, also stored,e.g., in memory 82. By anticipating a potentially incorrect ventriculartachyarrhythmia detection, processor 80 is able to intervene with vagalstimulation to reveal rapid ventricular conduction during atrialtachyarrhythmia masked as ventricular tachyarrhythmia before the deviceincorrectly diagnoses and treats patient 14. Conversely, processor 80need not intervene with vagal stimulation until an imminent detection isindicated with a threshold confidence by basing the anticipation of aventricular tachyarrhythmia on a number of events indicative of such acondition.

In some examples, IMD 16 may take certain precautions as a preconditionto delivering AV nodal vagal stimulation to heart 12 (238) to reduce theventricular rate response to an atrial tachyarrhythmia. In one example,processor 80 of IMD 16 controls sensing module 86 to measure the R-Rinterval of heart 12 to confirm that the depolarization rate ofventricles 28, 32 is in a range appropriate for intervening with vagalstimulation. In particular, processor 80 controls sensing module 86 tomeasure a number of R-R intervals of heart 12, from which processor 80determines a median R-R interval to confirm that the depolarization rateof ventricles 28, 32 is below a maximum threshold and above a minimumthreshold. For example, a supraventricular tachycardia for which AVnodal vagal stimulation may be employed is not likely if the ventricularcontraction rate is too low (i.e. RR median is too low), e.g. if therate is less than 240 bpm. Conversely, if the depolarization rate ofventricles 28, 32 is too slow (i.e. R-R median too high), theventricular conduction through the AV node during atrial tachyarrhythmiais likely not rapid enough to warrant attention, let alone interventionwith vagal stimulation. Therefore, processor 80 may not control signalgenerator 84 to deliver vagal stimulation, unless the median R-Rinterval is below the maximum threshold and above the minimum threshold.

In some examples, in the event a ventricular tachyarrhythmia is notdetected (232), an atrial tachyarrhythmia is detected (234), and animminent ventricular tachyarrhythmia detection is anticipated (236),processor 80 may control signal generator 84 to deliver vagalstimulation to patient 14 (238). In one example, processor 80 controlssignal generator 84 to employ one or more electrodes of RA lead 22 todeliver stimulation to or proximate to the AV node, e.g., to orproximate to the AV nodal vagal fat pad. Signal generator 84 deliversvagal stimulation in the form of bursts of pulses or a continuous trainof pulses. The stimulation may be delivered by signal generator 84according to one or more programmed stimulation parameters stored inmemory 82 including, e.g., amplitude, pulse width and frequency, as wellas the number of pulses within a burst. For example, signal generator 84may deliver vagal stimulation via electrodes on lead 22 with a frequencyin a range from approximately 20 Hz to approximately 100 Hz, andamplitude in a range from approximately 0.5 volts to approximately 8volts. Processor 80 controls signal generator 84 to deliver vagalstimulation to patient 14 (238) to block the AV node of heart 12, whichmay act to reduce ventricular conduction and contraction rate caused bya supraventricular tachycardia, e.g., during an atrial fibrillation.

IMD 16 may, in some examples, also be programmed with stimulationparameters configured to act as safety precautions to guard againstvagal stimulation preventing appropriate sensing in ventricles 28, 32.As signal generator 84 delivers high frequency stimulation, e.g., in theform of bursts of pulses to heart 12, blanking periods in which thedevice does not sense ventricular activity accumulate. There is a riskthat the blanking periods will accumulate beyond a threshold such thatsensing module 86 may not be able to detect the development of aventricular tachyarrhythmia, e.g. ventricular fibrillation in heart 12of patient 14. As such, in one example, delivering AV nodal, e.g. vagalstimulation to patient 14 (238) may include determining an amount oftime over which ventricular depolarizations need to be sensed during thedelivery of stimulation and delivering the stimulation based on thedetermined amount of time. FIG. 8 is a flowchart illustrating oneexample in which delivering AV nodal stimulation to patient 14 (238)includes determining an amount of time over which ventriculardepolarizations need to be sensed during the delivery of stimulation anddelivering the stimulation based on the determined amount of time.

In FIG. 8 , delivering AV nodal stimulation to patient 14 (238) includescalculating a median R-R interval (300), determining a sense time periodbased on the median R-R interval (302), delivering AV nodal stimulation(304), determining if a stimulation response time has ended (306) and,if the response time has not ended, continuing to deliver AV nodalstimulation (304). If, however, the stimulation response time has ended,AV nodal stimulation is thereafter delivered based on the sense timeperiod (308).

In one example of delivering AV nodal stimulation to patient 14 (238)according to the method of FIG. 8 , processor 80 of IMD 16 may beconfigured to monitor heart 12 over a period of time and store a numberof R-R intervals measured by sensing module 86 in memory 82. Based onthe measured R-R intervals, processor 80 may calculate a median R-Rinterval (300).

Processor 80 may determine a sense time period over which ventriculardepolarizations need to be sensed during the delivery of, e.g. AV nodalvagal stimulation by signal generator 84 based on the median R-Rinterval (302). In one example, processor 80 determines that the sensetime period needs to be a threshold percentage of the median R-Rinterval such that accumulated stimulation burst periods and associatedblanking periods do not subsume the period between ventricularcontractions. In one example, processor 80 determines that the sensetime period needs to be approximately 50% of the median R-R interval.

The method of delivering AV nodal stimulation to patient 14 (238)illustrated in FIG. 8 also includes, delivering AV nodal stimulationregardless of a measured sense time period (304) and determining, afterinitiating the stimulation, if a stimulation response period has ended(306). The stimulation response period is a period over which a minimumventricular rate response is expected and may be expressed, e.g. interms of a time or an absolute or relative response in the measuredventricular rate. As noted above, IMD 16 may be configured to allow theaccumulation of blanking periods for a brief period of time uponinitially delivering vagal stimulation to patient 14. As such, in oneexample, processor 80 controls signal generator 84 to deliver AV nodalstimulation for a brief period of time, in the example of FIG. 8 untilthe response time period ends, regardless of the actual sense timeperiod measured by the processor based on information from sensingmodule 86.

Once the response time period has ended (306), or some other allowabletime period over which violation of a threshold sense time period isacceptable, however, processor 80 may thereafter control signalgenerator 84 to deliver AV nodal, e.g. vagal stimulation to patient 14based on the determined sense time period (308). In one example,processor 80 controls signal generator 84 to deliver stimulation in theform of bursts, pulses, or trains of pulses such that each suchstimulation occurrence, e.g. one burst occurs of a time period that doesnot exceed approximately 50% of the median R-R interval, which, in turn,permits sensing module 86 to sense ventricular depolarizations over asense time period equal to at least approximately 50% of the median R-Rinterval. FIG. 9 is a graph illustrating the median R-R interval forheart 12 of patient 14 and the delivery of AV nodal stimulation inaccordance with the example of FIG. 8 by first delivering stimulationregardless of a measured sense time period (304), and, once astimulation response period has ended (306), delivering stimulation tothe patient based on the determined sense time period (308).

In FIG. 9 , processor 80 controls signal generator 84 to deliver aseries of, e.g. AV nodal vagal stimulation bursts 400 to heart 12 ofpatient 14 over a total stimulation time period. Upon initiating vagalstimulation before a stimulation response time period ends, processor 80controls signal generator 84 to deliver vagal stimulation regardless ofthe quantity of the sense time period over which it is possible forsensing module 86 to sense ventricular depolarizations, e.g. for thepurposes of detecting a serious arrhythmia in ventricles 28, 32, e.g.ventricular fibrillation. As such, as illustrated in the example of FIG.9 , processor 80 may control signal generator 84 to deliver vagalstimulation before the response time period ends even though thestimulation burst time period is more than approximately 50% of themedian R-R interval, which limits the sense time period to less than thethreshold approximately 50% of the median R-R interval. After thestimulation response time period ends, processor 80 may thereaftercontrol signal generator 84 to deliver vagal stimulation based on atarget sense time period, e.g. a sense time period greater than or equalto approximately 50% of the median R-R interval. As such, as illustratedin FIG. 9 , after the stimulation response time period ends, the bursttime period for bursts delivered by signal generator 80 may be less thanor equal to approximately 50% of the median R-R interval, which willpermit the sense time period to be greater than or equal to thethreshold approximately 50% of the median R-R interval.

Although the example of FIG. 9 is illustrated in terms of sense timeperiods for a single burst cycle of vagal stimulation between successiveventricular depolarizations, as noted above, in some examples, thethreshold for the sense time period may be a percentage of the totalstimulation time period such that the accumulation of blanking periodsduring the total stimulation time period does not exceed the thresholdpercentage. In one example, the threshold is 50% such that during 30seconds of vagal stimulation a window of at least 15 seconds in whichsensing module 86 may sense activity in ventricles 28, 32 is needed.

Referring again to the example method of FIG. 8 , in some examples, IMD16 may be configured to synchronize the delivery of AV nodal stimulationwith a QRS complex of heart 12. In particular, IMD 16 may be configuredto deliver the vagal stimulation in a refractory period betweendepolarization/repolarization cycles. During the refractory period, thestimulation is less likely to depolarize heart 12, and, in particular,ventricles 28, 32.

IMD 16 continues to deliver AV nodal stimulation until one or morestimulation termination criteria are satisfied (240), at which point thedevice terminates the stimulation (242). In one example, the terminationcriteria includes at least one of expiration of a programmed stimulationdelivery time period, an accumulation of blanking time periods thatexceeds a threshold percentage of a stimulation delivery time period,failure to detect a threshold ventricular rate response within astimulation response time period, or detection of a ventriculartachyarrhythmia.

In some examples, IMD 16 is programmed, e.g. according to a therapyprogram stored in memory 82 to deliver AV nodal vagal stimulation for aspecific period of time. The programmed stimulation time period may beset to a value that provides a sufficient amount of time for IMD 16 totest the effectiveness of the stimulation in modulating the ventricularrate response. Additionally, regardless of other termination criteria,the stimulation time period may be set to a value that provideshysteresis such that IMD 16 is not rapidly toggling between turningvagal stimulation on and off. In one example, processor 80 of IMD 16 isprogrammed with a vagal stimulation time period stored in memory 82 in arange from approximately 20 seconds to approximately 30 seconds, uponthe expiration of which processor 80 is programmed to terminate deliveryof vagal stimulation to heart 12 of patient 14 (242).

In one example, processor 80 may be programmed to control sensing module86 to monitor the stimulation delivered by signal generator 84 and theperiods of time sensing module 86 is sensing activity in ventricles 28,32 during stimulation to ensure that an accumulated blanking period doesnot exceed a threshold percentage of a stimulation delivery time periodstored in memory 82. In the event the blanking periods exceed athreshold, which corresponds to a sense time period being less than athreshold, processor 80 may be programmed to control signal generator 84to terminate stimulation. In some examples, however, processor 80 may beprogrammed to allow the blanking period, which may generally correspondto the stimulation burst period to exceed a threshold percentage of thecontraction frequency in ventricles 28, 32 for a brief period at thebeginning of the delivery of AV nodal stimulation before the effect ofthe stimulation is able to slow the contractions of ventricles 28, 32.

In addition to stimulation time period expiration and blanking periodaccumulation, processor 80 may be programmed to control signal generator84 to terminate the delivery of AV nodal stimulation (242) in the eventa minimum ventricular rate response is not observed within a programmedresponse time period. In one example, processor 80 may be programmed toterminate AV nodal vagal stimulation if the contraction rate ofventricles 28, 32 does not decrease by a threshold amount within thestimulation response time period. The minimum ventricular rate responsemay differ from one patient to another and may be set as a relativepercentage reduction or as an absolute value rate reduction. In examplesin which the minimum rate response is set as an absolute value, thevalue by which processor 80 measures rate response may be tiereddepending on the observed contraction rate of ventricles 28, 32, i.e.the minimum rate response may be higher for higher initial ventricularcontraction rates and lower for lower ventricular contraction rates.

In one example, processor 80 is programmed with a vagal stimulationresponse time period of approximately 10 seconds or in a range of 5 to10 beats of heart 12. Additionally, processor 80 is programmed with aminimum rate response approximately equal to a 20% rate reduction fromthe initial contraction rate of ventricles 28, 32 measured by sensingmodule 86. In another example, IMD 16 is programmed with a minimum rateresponse approximately equal to 40 bpm for higher initial ventricularcontraction rates on the order of 180 bpm or higher, and a minimum rateresponse approximately equal to 20 bpm for lower initial ventricularcontraction rates on the order of 120 bpm or lower. The stimulationresponse time period and minimum ventricular rate response values, whichare employed by processor 80 as basis for controlling signal generator84 to terminate stimulation (242), may, in some examples, be stored inmemory 82 of IMD 16.

Various examples have been described that include providing AV nodalstimulation during atrial tachyarrhythmia to control ventricular rateresponse and thereby prevent ventricular tachyarrhythmia misdiagnosisand treatment delivery. In one example, AV nodal vagal stimulation mayinclude stimulation of the AV nodal region of a patient's heart through,e.g., a single endocardial screw-in lead that provides atrialpacing/sensing as well as the AV nodal vagal stimulation. While theforegoing examples are described with reference to preventing orreducing the risk of delivering inappropriate shocks to a patient, thereare a number of other applications for examples according to thisdisclosure. First, by better controlling the ventricular rate during AF,patient symptoms may be reduced. Secondly, by preventing rapidlyconducted AF, it may be possible to deliver a greater percentage ofbi-ventricular pacing therapy to CRT patients. These and other examplesare within the scope of the following claims.

The invention claimed is:
 1. A system comprising: a stimulationgenerator configured to deliver electrical stimulation to a patient; asensing module configured to sense activity of the patient's heart; anda processor coupled to the sensing module and configured to: anticipatea ventricular tachyarrhythmia in a heart of a patient based on theventricular tachyarrhythmia not yet being detected but a counterreaching a threshold value of a ventricular tachyarrhythmia event count;determine a sense time period; and in response to anticipating theventricular tachyarrhythmia, control the stimulation generator todeliver electrical stimulation to block the atrioventricular node of theheart over an electrical stimulation delivery time period, wherein theelectrical stimulation delivery time period is based on the sense timeperiod over which ventricular depolarizations can be sensed during theelectrical stimulation delivery time period, the electrical stimulationdelivery time period being bounded by limits that prevent anaccumulation of blanking periods applied to a sensing amplifier thatwould exceed the sense time period.
 2. The system of claim 1, whereinthe processor is configured to detect an atrial tachyarrhythmia, andanticipate, during the detected atrial tachyarrhythmia, detection of theventricular tachyarrhythmia in the heart of the patient based on theventricular tachyarrhythmia not yet being detected but the counterreaching the threshold value of the ventricular tachyarrhythmia eventcount.
 3. The system of claim 1, wherein the processor is configured todetect an atrial tachyarrhythmia comprising at least one of atrialfibrillation or atrial tachycardia.
 4. The system of claim 3, whereinthe processor detecting the atrial tachyarrhythmia comprises: detectinga P-P interval value for the heart of the patient that is less than apercentage threshold of an R-R interval value; and analyzing a rhythm ofthe heart of the patient for indications of sinus tachycardia.
 5. Thesystem of claim 1, wherein the processor is configured to anticipate aventricular tachyarrhythmia based at least on the threshold value of theventricular tachyarrhythmia event count.
 6. The system of claim 5,wherein the ventricular tachyarrhythmia event count comprises a numberof sensed ventricular intervals indicative of the ventriculartachyarrhythmia.
 7. The system of claim 1, wherein the processor isconfigured to control the stimulation generator to deliver theelectrical stimulation during a refractory period of the heart of thepatient.
 8. The system of claim 1, wherein the processor is configuredconfirm, prior to controlling the stimulation generator to deliver theelectrical stimulation, that a median RR interval for the heart of thepatient is less than a maximum threshold ventricular contraction rateand greater than or equal to a minimum threshold ventricular contractionrate.
 9. The system of claim 1, wherein the processor is configured todetermine the sense time period based on an R-R interval of the heart ofthe patient.
 10. The system of claim 9, wherein the processordetermining the sense time period based on an R-R interval of the heartof the patient comprises: measuring a plurality of R-R intervals for theheart of the patient; calculating a median R-R interval from themeasured R-R intervals; and setting the sense time period to greaterthan or equal to a predetermined portion of the median R-R interval. 11.The system of claim 1, wherein the processor is configured to controlthe stimulation generator to terminate the delivery of electricalstimulation based on one or more stimulation termination criteria,wherein the one or more stimulation termination criteria comprises atleast one of expiration of an electrical stimulation delivery timeperiod, an accumulation of blanking time periods exceeding a thresholdpercentage of the electrical stimulation delivery time period, failureto detect a threshold ventricular rate response within an electricalstimulation response time period, or detection of a ventriculartachyarrhythmia.
 12. The system of claim 9, wherein the processordetermining the sense time period using on an R-R interval comprises:measuring a plurality of R-R intervals for the heart of the patient;calculating a median R-R interval from the measured R-R intervals; andsetting the sense time period to greater than or equal to apredetermined portion of the median R-R interval.
 13. A systemcomprising: a stimulation generator configured to deliver electricalstimulation to a patient; and a processor configured to: increment acounter upon detection of a ventricular tachyarrhythmia event;anticipate detection of a ventricular tachyarrhythmia in a heart of apatient based on the ventricular tachyarrhythmia not yet being detectedbut a counter reaching a threshold value of a ventriculartachyarrhythmia event count; measure an RR interval; determine, usingthe RR interval, a sense time period over which ventriculardepolarizations need to be sensed in order to detect development of theventricular tachyarrhythmia; determine a stimulation time period basedon the sense time period; in response to anticipating detection of theventricular tachyarrhythmia, control the stimulation generator todeliver electrical stimulation to block an atrioventricular node of theheart over a first time period regardless of the sense time period; andcontrol the stimulation generator to deliver the electrical stimulationduring the stimulation time period over a second time period after thefirst time period, the stimulation time period being bounded by limitsthat prevent an accumulation of blanking periods applied to a sensingamplifier that would exceed the sense time period.
 14. The system ofclaim 13, wherein the processor is configured to detect an atrialtachyarrhythmia, and anticipate during the detected atrialtachyarrhythmia, detection of the ventricular tachyarrhythmia in theheart of the patient based on the ventricular tachyarrhythmia not yetbeing detected but the counter reaching the threshold value of theventricular tachyarrhythmia event count.
 15. The system of claim 13,wherein the processor is configured to detect an atrial tachyarrhythmiacomprising at least one of atrial fibrillation or atrial tachycardia.16. The system of claim 15, wherein the processor detecting the atrialtachyarrhythmia comprises: detecting a P-P interval value for the heartof the patient that is less than a percentage threshold of an R-Rinterval value; and analyzing a rhythm of the heart of the patient forindications of sinus tachycardia.
 17. The system of claim 13, whereinthe processor is configured to anticipate a ventricular tachyarrhythmiabased at least on the threshold value of the ventricular tachyarrhythmiaevent count, and wherein the ventricular tachyarrhythmia event countcomprises a number of sensed ventricular intervals indicative of theventricular tachyarrhythmia.
 18. The system of claim 13, wherein theprocessor is configured to control the stimulation generator to deliverthe electrical stimulation during a refractory period of the heart ofthe patient.
 19. The system of claim 13, wherein the processor isconfigured confirm, prior to controlling the stimulation generator todeliver the electrical stimulation, that a median RR interval for theheart of the patient is less than a maximum threshold ventricularcontraction rate and greater than or equal to a minimum thresholdventricular contraction rate.
 20. The system of claim 13, wherein theprocessor is configured to control the stimulation generator toterminate the delivery of electrical stimulation based on one or morestimulation termination criteria, wherein the one or more stimulationtermination criteria comprises at least one of expiration of anelectrical stimulation delivery time period, an accumulation of blankingtime periods exceeding a threshold percentage of the electricalstimulation delivery time period, failure to detect a thresholdventricular rate response within an electrical stimulation response timeperiod, or detection of a ventricular tachyarrhythmia.